Question: For real numbers $t,$ the point
\[(x,y) = (\cos^2 t, \sin^2 t)\]is plotted.  All the plotted points lie on what kind of curve?

(A) Line
(B) Circle
(C) Parabola
(D) Ellipse
(E) Hyperbola

Enter the letter of the correct option.
Solution: Since $\cos^2 t + \sin^2 t = 1,$ all the plotted points lie on the line $x + y = 1.$  The answer is $\boxed{\text{(A)}}.$